Aqueous dispersions of polymer particles are used extensively in waterborne paint products. A common requirement within typical formulating practices for such waterborne paints is that the use of rheological additives and other modifiers is relied upon to a much greater extent than for their solvent based counterparts. Depending on the end use, it is not uncommon for waterborne paints to contain more than one additive to provide the balance of viscosities required for application, anti-settling and flow characteristics and to optimise film formation.
Common rheological control additives include thickening agents such as cellulose ethers and water-soluble gums. Thickening agents of this class often rely on flocculation to impart their thickening effect, which in turn can have detrimental effects on other properties of the paint. Accordingly, paints formulated with such thickening agents can exhibit rheology conducive to easy application, but at the same time demonstrate inferior package homogeneity, leveling, gloss, bonding ability to chalked paint surfaces and exterior durability.
Other rheological control additives commonly used are the so called alkali swellable acrylic (ASA) thickeners, which are acid containing acrylic latex copolymers. When neutralised with base such as ammonia, these materials become thick, viscous liquids. In particular, upon being neutralised the particles of acrylic copolymer that make up the thickener absorb water and swell to thereby increase the viscosity of the material. Such materials are typically employed at relatively low levels and can effectively increase the viscosity of paint formulations. However, to achieve their thickening effect the acrylic copolymer particles must be prepared using relatively high levels of acid monomer, the presence of which can render paint films water sensitive. Accordingly, it is generally impractical to incorporate ASA thickeners at high levels in paint formulations or use them as a sole thickener/binder in most applications. Furthermore, this class of thickener is prone to being sensitive to other formulation ingredients such as the addition of water miscible co-solvents associated with tinters or other additives.
Rheological control additives play an important role in enabling paints to be formulated with desirable rheological properties. However, the use of such additives, in particular thickening agents, can be problematic in that they can impart undesirable characteristics to the paint and/or the resulting paint film, and increase the complexity of formulating. Considerable research effort has therefore been directed toward developing paint formulations that provide suitable rheological properties with less reliance on rheological control additives.
One area of research has concentrated on developing means to reduce the amount of thickening agent required in paint formulations. An approach that has met with some success has been to use a specific class of aqueous dispersions of polymer particles which have an alkali responsive thickening effect similar to that of the ASA thickeners. However, unlike the ASA thickeners, these aqueous dispersions have a limited ability to thicken, and they can effectively be used as the sole binder in paint formulations. In particular, the aqueous dispersions are prepared using less acid monomer which on one hand reduces the aforementioned water sensitivity problem, and on the other hand renders the polymer particles with less ability to swell and thereby minimises the thickening effect of the dispersion.
U.S. Pat. No. 3,875,099 discloses a method for preparing an aqueous dispersion of swellable acrylic copolymer particles. The particles contain 0.5 to 2.5 weight % of an α,β-unsaturated carboxylic acid, and the particular swelling characteristics of the particles are said to result from the controlled introduction of the acid during the polymerisation process. In particular, at least 70 weight % of the acid must be introduced during the process in a manner such that the acid monomer portion is 3 to 15 weight % by weight of all the monomers being concurrently added, such addition further beginning before more than 50 weight % of the non-acid monomers have been introduced and terminating before more than 90 weight % of the non-acid monomers have been introduced.
U.S. Pat. No. 4,468,498 discloses a process for making an aqueous dispersion of swellable water insoluble core/sheath polymer particles. In this case, the particular swelling characteristics of the particles are said to result from an ionic core polymer containing ionisable acid groups. In particular, the core polymer has a composition comprising a non-acid monomer copolymerised with at least 5 weight % of a carboxylic acid monomer, or 15 to 100 weight % of the acid monomer where no other hydrophilic monomer is present, based on the weight of monomers used to form the core polymer. A sheath polymer, which constitutes at least four times the mass of the core polymer and optionally contains low levels of acid monomer, encapsulates the core polymer to thereby form the particles.
As mentioned above, aqueous dispersions that are prepared using lower acid monomer levels, such as those disclosed in U.S. Pat. No. 3,875,099 and U.S. Pat. No. 4,468,498, exhibit a reduced thickening effect compared with ASA thickeners, but are more suited for use as a sole binder in paint formulations. Despite such aqueous dispersions exhibiting a reduced thickening effect, the degree of thickening that they can ultimately impart to a given paint formulation nevertheless still needs to be considered carefully. Inadequate swelling of the polymer particles can result in little if no thickening effect being imparted to the formulation, and a relatively large degree of swelling of the polymer particles can render the formulation highly viscous. While it is desirable to increase the viscosity of the formulation, to date such highly viscous formulations have had limited practical use in coating applications. In particular, highly viscous paint formulations laden with large swollen polymer binder particles typically have poor flow and leveling properties, the effect of which often results in applicator marks, such as brush lines, being retained in the cured paint film. Gloss properties of a resultant paint film are also typically greatly impaired.
Accordingly, aqueous dispersions of swellable polymer particles suitable for use as sole or primary binders in paint formulations are typically prepared such that the polymer particles can swell to impart some thickening effect, yet are also restricted from swelling to an extent that would render a formulation highly viscous. The polymer particles disclosed in U.S. Pat. No. 4,468,498 for example are prepared such that they comprise a swellable core of concentrated polymeric acid which is encapsulated by a sheath polymer. A crosslinking monomer is typically included in the core polymer to restrict its flexibility and thereby limit its ability to swell. Paints formulated using such a material as the sole binder are said to require less thickening agent than would typically be required if a conventional acrylic latex binder were used.
The aqueous dispersion of polymer particles disclosed in U.S. Pat. No. 3,875,099 is prepared such that the polymer particles comprise a reduced level of acid monomer compared with U.S. Pat. No. 4,468,498, and generally a considerable amount of hydrophobic monomer. The combined effect of the hydrophobic monomer content and the reduced level of acid monomer is believed to render these polymer particles with a limited ability to swell. Aqueous dispersions of such polymer particles are said to be capable of achieving viscosities of up to 30,000 centipoises at a solids content of about 50%. Paints formulated using these aqueous dispersions as a sole binder are also likely to require less thickening agent than would typically be required if a conventional acrylic latex binder were used.
Despite the advantage of being able to reduce the amount of thickening agent employed in paint formulations through use of state of the art aqueous dispersions of swellable polymer particles, it would be desirable to provide an aqueous dispersion of polymer particles that could be used to prepare versatile paint formulations which required little if no thickening agent. It would also be desirable if such aqueous dispersions could render products, such as paints, with a relatively high viscosity, yet still enable the product to demonstrate good levelling, flow and gloss properties.